The production and use of polysuccinimide and polyaspartic acid and derivatives thereof has long been the subject of numerous publications and patents. Thus, they can be produced by thermal polycondensation of aspartic acid (J. Org. Chem. 26, 1084 (1961)).
U.S. Pat. No. 4,839,461 (=EP-A 0 256 366) describes the production of polyaspartic acid from maleic anhydride, water and ammonia. Maleic anhydride is converted into the monoammonium salt in aqueous medium by addition of concentrated ammonia solution. The monoammonium salt is melt-polymerized to polysuccinimide and converted by hydrolysis into PAA or a salt thereof, see also WO 93/23452.
It is known from U.S. Pat. No. 4,590,260 that amino acids can be subjected to polycondensation together with derivatives of malic, maleic and/or fumaric acid at temperatures of 100.degree. to 225.degree. C. According to U.S. Pat. No. 4,696,981, microwaves are used for such reactions.
DE-A 2 253 190 (=U.S. Pat. No. 3,846,380) describes a process for the production of polyamino acid derivatives, particularly polyaspartic acid derivatives, which--in addition to aspartic acid--uses maleic acid derivatives (monoammonium salt and monoamide) to produce the polysuccinimide by thermal polymerization; the polysuccinimide may then be converted into the required derivatives by reaction with amines in suitable solvents.
The reaction of maleic anhydride and ammonia to form maleic amide acid is also known. As disclosed in U.S. Pat. No. 2,459,964, maleic amide acid is obtained by introduction of ammonia into a solution of maleic anhydride in xylene, ethylene chloride, tetralin or dioxane at 50.degree. to 100.degree. C. and isolation of the amide acid precipitated by filtration.
DE-A 945 987 describes a process in which maleic anhydride is reacted with ammonia in the presence or absence of water to form maleic amide acid. In variants of this process, powdered maleic anhydride is introduced into a cooled 21% aqueous ammonia solution or powdered maleic anhydride is treated with gaseous ammonia.
The above-described processes which start out from maleic anhydride and ammonia are all attended by the disadvantage that they take place in solution, so that the reaction products have to be separated from the solution, or that they involve the handling and mixing of powder-form maleic anhydride which involves industrial hygiene problems and is complicated in terms of process technology.